I had a NOS Tesla EL34 short and go up in smoke and it bit my TORII MK3 and she needed to go for a trip to visit Steve, Sara and the gang. Figured why it was in Illionios up on the lift we might as well bring her up to spec upgrade to a MK4. Any of you folks have the upgrade done yet? Was wondering/hoping if anyone could give me some feedback. Is it a big improvement? Did you stay with EL34's or did start rolling and trying all the new flavors now available to us? Any tips or ideas or feedback would really be appreciated. Thanks

I have done only a small amount of experimenting with the MK IV but I can tell you that this amp responds vividly to the smallest change. I pre-bought the Tung Sol KT66s to have on hand while waiting for the amp. Those tubes sounded better to me than the stock EL34s. They seem to be better balanced and behaved. I have kept them in the amp since. I will go back to the 34s some day because I may have been hasty to dismiss them as quickly as I did.

I thought I had a defective rectifier because I was getting noticible ringing from one that I could hear across the room during quiet moments. Before I sent it in I purchased a pair of Philips 5R4GYS from Upscale Audio. Upscale was over the top with their praise for these tubes. The original issue since went away when I tried swapping the rectifiers across channels, which I should have done first. I now had the Philips so I gave them a try. I really do like these Philips tubes better. Having the stock rectifiers as backup is fine.

I may want to try some different KT66s at some point. These seem to fit the design of the amp the best.